In general, a Doherty amplifier has a structure of connecting a carrier amplifier and a peaking amplifier in parallel using a quarter-wave (λ/4) transformer line. With an increase of a power level, an amount of current supplied to a load by the peaking amplifier increases, and performance increases through the control of the load impedance of each of the carrier amplifier and the peaking amplifier.
A microwave Doherty amplifier was proposed by ‘W. H. Doherty’ in 1936. At the beginning, the microwave Doherty amplifier was devised as an Amplitude Modulation (AM) transmitter of a broadcasting device that makes use of a Low Frequency (LF) vacuum tube or a Medium Frequency (MF) vacuum tube. Since then, there have been several proposals for realization of a Doherty amplifier using a solid-state device not a vacuum tube, and many studies have been made for substantial realization.
A power amplifier through asymmetric power coupling called the Doherty amplifier achieved high performance and high linearity. Particularly, there was much performance improvement for a Doherty amplifier of a Base Station (BS) and a Mobile Station (MS) of a mobile communication system. A Doherty amplifier at a high frequency band is composed of an input power divider, a transmission line for synchronizing a phase between carrier/peaking amplifiers, the carrier/peaking amplifiers realized to output the same value while providing the maximum output in one amplifier by constructing an input/output matching circuit of each amplifier, and a quarter-wave transmission line for offset line and Doherty operation for, when the peaking amplifier does not operate, increasing an output impedance and inducing the occurrence of a suitable load modulation phenomenon.
The above construction is to enable matching of not only a real part but an imaginary part by disposing a matching circuit in an output unit of a transistor and disposing an offset line after the matching circuit, thereby obtaining the maximum output of an amplifier and simultaneously inducing a Doherty operation. Also, studies were made for an N-way Doherty amplifier that is a structure capable of further generalizing a Doherty amplifier and optimizing performance and linearity. In addition, studies were made for an N-stage Doherty power amplification scheme of gradually inducing high performance from a power level lower than that of a general Doherty amplifier. Alternatively, a Doherty amplifier using an envelope tracking device has been realized to solve a problem of failing to provide the maximum output due to a low bias of a peaking amplifier. Studies were conducted for a Doherty amplifier using asymmetric power driving that changes input power dividing.
As described above, various Doherty power amplification technologies were developed. But as a BS and an MS of a mobile communication system are gradually miniaturized and the necessity of price reduction increases, there is a demand for higher performance than that of a conventional Doherty amplifier, and particularly, high performance at average output power at the time of applying a modulation signal. FIG. 1 illustrates a performance characteristic of a class B amplifier, and a Doherty amplifier, and a schematic performance characteristic of a Doherty amplifier whose actual realization is easy in realizing an ideal Doherty amplifier in view of performance, and a probability density function and power generation function of World Interoperability for Microwave Access (WiMAX) for wireless communication in a BS with a Peak to Average Power Ratio (PAPR) of 7.8 dB. In FIG. 1, the Doherty type I illustrates performance decreases by unsuitable load modulation at maximum power regions of a carrier amplifier and a peaking amplifier. Also, the Doherty type II illustrates performance decreases at a region backed off by the effects of a leakage of carrier output power capable of occurring because the output impedance of a peaking amplifier is less, or a performance decrease occurring because a carrier amplifier does not perform sufficient saturation operation due to On-resistance. When a modulation signal is applied regarding each performance characteristic, performance at an average power region backed off as much as a PAPR can be determined according to Equation 1 below. The determined result is given according to Table 1 below.
                              η          avg                =                                            ∫              0                              P                                  out                  ,                  max                                                      ⁢                                          p                ·                                  ⅆ                                      ·                                          f                      ⁡                                              (                                                  P                          out                                                )                                                                                                        ⁢                              P                out                            ⁢                              ⅆ                                  P                  out                                                                                        ∫              0                              P                                  out                  ,                  max                                                      ⁢                                          p                ·                                  ⅆ                                      ·                                          f                      ⁡                                              (                                                  P                          out                                                )                                                                                                        ⁢                                                P                  dc                                ⁡                                  (                                      P                    out                                    )                                            ⁢                              ⅆ                                  P                  out                                                                                        [                  Eqn          .                                          ⁢          1                ]            
In Equation 1 above, the ‘ηavg’ represents an average performance, the ‘Pout,max’ represents the maximum output power, the ‘Pout’ represents output power, and the ‘pdf(Pout)’ represents a probability distribution function of the output power.
TABLE 1Class BIdeal DohertyType IType IIPerformance31.6%61.6%59.4%56.3%
As shown in Table 1, it can be appreciated that the Doherty type I has performance almost similar to that of the ideal Doherty amplifier and, through this, performance at a back-off region has great effect on the whole performance characteristic compared to performance at a maximum power region.